Since the 1960's, the iron and steel industry has rapidly developed with increasing demand for iron and steel production in accordance with rapid economic growth. According to the Korea Iron and Steel Association, domestic crude steel production capacity exhibits a steady increase each year, in spite of a slight decrease in 1998 due to the IMF financial crisis, up to approximately 53 million tons in 2008 making Korea the world's sixth largest steelmaker.
The iron and steel industry consumes large quantities of raw materials and energy and generates a large quantity of steel slag as a by-product through complicated production processes such as iron-making, steel-making, rolling, and the like. The steel slag is classified into blast furnace slag and steel-making slag. The blast furnace slag is classified into water-cooled slag and slow-cooled slag (air-cooled slag), and the steel-making slag is classified into converter furnace slag and electric arc furnace slag. The electric arc furnace slag is classified into oxidized slag and reduced slag.
The steel-making slag is a more lightweight material than iron and is substantially separated by a difference in specific gravity, thereby hardly containing heavy metals. Accordingly, diverse research has been recently conducted into the steel-making slag as a construction material due to low environmental impact. However, since the steel-making slag contains free-calcium oxide (free-CaO), a chemical reaction occurs when the steel-making slag contacts water, thereby causing volumetric expansion. Thus, when the steel-making slag is used for roads or concrete, cracks occur. In this case, a method of using steel-making slag chemically stabilized by a post-treatment such as an aging process has been proposed. However, it is still low in reliability, so that its application is rare in practice.
In order to commercialize steel-making slag, a method of controlling an amount of free-CaO generated by rapidly cooling molten steel-making slag using high-speed air has recently been developed. Steel-making slag produced by this method is called either atomized steel slag (ASS), since the steel-making slag is spherical, or rapidly cooled steel slag, since the steel-making slag is obtained by a rapid cooling process.
The atomized steel slag has a low possibility of expansion collapse due to a low content of free-CaO. In addition, since the atomized steel slag is formed in a fine aggregate form which has a near spherical particle shape, the atomized steel slag used as a concrete construction material increases fluidity by a ball bearing effect. However, the atomized steel slag increases a possibility of segregation due to higher density thereof than other materials constituting the concrete, so that it is rarely applied to other general usages than special concrete due to the segregation possibility.
The amounts of steel slag produced in Korea were 16.62 million tons in 2006, 17.53 million tons in 2007, and 18.67 million tons in 2008, increasing by about 1 million tones every year. Such continuous increase in production of steel slag in recent years results from starting of the operation of Posco's Finex plant and an electric arc furnace of a hot-rolled steel mill by Hyundai Steel in Dangjin (The Korea Iron and Steel Association, 2008).
It was reported that the amount of recycled steel slag was 18.61 million tons in 2007, which was 99.7% of the total production of steel slag. The amounts of produced blast furnace slag and converter furnace slag were respectively 9.50 million tons and 5.40 million tons, and blast furnace slag and converter furnace slag were 100% recycled. The amount of recycled electric arc furnace slag was 3.707 million tons, which is 98.4% of the total production. Accordingly, it seems that recycling of steel slag has been efficiently carried out.
When the degree of recycling steel slag is evaluated, over 80% of blast furnace slag has been used to produce higher value-added products such as raw materials for cement, fertilizers, and the like, as a result of intensive efforts devoted to develop use of the blast furnace slag for recycling for a long period of time. Thus, it may be considered that the degree of recycling the blast furnace slag is relatively high. However, approximately 80% of steel-making slag has been used only for lower value-added construction aggregate. Thus, it may be considered that the degree of recycling the steel-making slag is relatively low. When the steel-making slag is used as the construction aggregate, a time-consuming aging process is required. Therefore, costs for recycling the steel-making slag increase.
On the other hand, electric arc furnace slag refers to industrial wastes discharged from a converter furnace or an electric arc furnace in which steel-making raw materials such as pig iron and scrap iron are refined. When the electric arc furnace slag is dumped into a landfill without being recycled, not only environmental problems such as fugitive dust and landfill leachate, but also economical problems such as securing of a large area landfill are caused. Thus, diverse research has been conducted into recycling of electric arc furnace slag. As a result of such efforts, Korean Industrial Standards (KS) for electric arc furnace oxidized slag fine aggregate have been established.
Although not statistically studied yet, electric arc furnace reduced slag accounts for approximately 20% of electric arc furnace slag. The amounts of the electric arc furnace reduced slag based on this rate are estimated to be about 0.75 million tons in 2007 and up to about 1 million tons in 2010. However, suitable recycling use for the electric arc furnace reduced slag has not been found, and thus the degree of recycling the electric arc furnace slag is reduced thereby. Thus, there is still a need to develop techniques of recycling the electric arc furnace reduced slag. However, due to a high content of free-calcium oxide, up to about 20%, contained in the electric arc furnace reduced slag, higher value-added products manufactured form the electric arc furnace reduced slag have yet to be reported.
Korean Patent Application Publication No. 10-2009-0070404 discloses a method of manufacturing a blended cement composition by adding fly ash and ordinary Portland cement to slow-cooled reduced slag as a method of recycling reduced slag.
In addition, U.S. Pat. No. 6,033,467 discloses a method of making cement using slag recovered from smelters of nickel, copper, lead, or zinc. However, this method is focused not on the production of blended cement but on a method of using waste to remove environmental contaminants. Therefore, products obtained from this method do not have particular advantageous characteristics.
U.S. Pat. No. 6,776,839 discloses blended cement having increased strength through Pozzolanic reaction with slag. However, the blended cement has a lower strength than ordinary Portland cement. This is because slow-cooled slag cannot have early hydraulicity as known in the art.
Korean Patent Application Publication No. 10-2002-0039520 discloses a method of preparing a non-sintered cement using blast furnace slag as a main material to improve a low early strength of cement, thereby replacing ordinary Portland cement.
Differently from the present invention, the above-mentioned documents disclose electric arc furnace reduced slag, which is slowly and sufficiently cooled, and also disclose a grinding process and a fabricating process after a cooling process. According to such conventional methods, space for cooling the electric arc furnace reduced slag is further required, and stability of free-calcium oxide after cooling needs to be secured. In order to overcome these problems, these documents also propose use of electric arc furnace reduced slag mixed with other waste materials or other additives. However, a manufacturing process is complicated, and manufacturing costs therefor increase according to these methods.
Meanwhile, cement and application products thereof generally harden at around 28 days, and desired properties thereof are achieved thereafter. Thus, rapid hardening cement and application products thereof are used for urgent construction of roads, bridges, harbors, sewer pipes, and the like.
It is known that rapid hardening cement is generally prepared by mixing clinker, which includes rapid hardening minerals, such as CaO.Al2O3, 12CaO.7Al2O3, and 11CaO.7Al2O3.CaX (X: halogen element), with gypsum, and grinding the mixture, or by mixing powder of the rapid hardening minerals with ordinary Portland cement, gypsum, and other additives. (Korean Patent Publication Nos. 76-397 and 90-33 and Japanese Patent Application Publication Nos. sho 52-139819, 63-285114, and 64-37450)
However, physical properties of the above-mentioned rapid hardening cement vary depending on time of manufacture due to high manufacturing costs for clinker in a high-temperature sintering kiln and difficulty in controlling volatile components or melting components. Particularly, the rapid hardening cement varies in volume due to crystallization of ettringite (3CaO.Al2O3.3CaSO4.32H2O) caused by a high content of Al2O3 which is a main hydrous mineral exhibiting rapid hardening among hydrated compounds produced via reaction between cement and water, has decreased stability against water due to gel hydrates of Al(OH)3, and expands in volume through reaction with SO4 ions in the presence of sulfates. As a result, deterioration in structural stability has been regarded as problematic.
Methods of mixing ground Hauynite type clinker including calcium sulfo aluminate, as a main component, with ordinary Portland cement, gypsum, calcium hydroxide, and the like have been introduced (Korean Patent Publication Nos. 97-008685, 10-0220340, and 10-0310657), as methods of overcoming such performance drawbacks of cement and improving structural stability thereof after hardening.
Furthermore, Korean Patent No. 0310657 discloses a method of manufacturing rapid hardening cement. Korean Patent No. 0670458 discloses a method of manufacturing mortar utilizing rapid hardening cement. Korean Patent No. 0755272 discloses a method of manufacturing rapid hardening cement and latex concrete.
Generally, rapid hardening cement reacts with water to harden within several to several tens of minutes during the manufacture of mortar or concrete, for example, to have a strength of 20 MPa or more within 3 to 6 hours and forms a cement structure at an early stage. Thus, deformation caused by long-term evaporation of water may be minimized, and a stable structure in which cracks hardly occur may be formed. Accordingly, the rapid hardening cement is widely used for emergency repair of roads, bridges, and the like. However, most mortar developed to date does not utilize rapid hardening cement. Generally, most mortar-developing companies add functional ingredients to mortar to improve properties of mortar due to limits of technology for the rapid hardening cement. Thus, the development of a hydraulic binder having rapid hardening properties and not including functional ingredients is required.